Process for aluminizing ferrous metals



United States Patent 3,212,923 PROCESS FOR ALUMINIZING FERROUS METALSGeorge V. Sneesby, Canoga Park, Calif., assignor to North AmericanAviation, Inc. No Drawing. Filed Nov. 30, 1962, Ser. No. 241,151 7Claims. (Cl. 117-119) My invention relates to a method of providing aprotec tive aluminum surface on ferrous metals, and more particularly toan improved method of providing a protective diffusion coating ofaluminum on ferrous metals from an alkali metal bath.

The co-pending application of the common assignee, Serial No. 377,507filed June 24, 1964 in the name of Roger D. Moeller forAluminum-Containing Diifusion Coating for Metals, which is acontinuation-in-part of Serial No. 85,457 filed January 30, 1961, nowabandoned, discloses a method of providing protective diffusion layercoatings on base metals from molten alkali metal baths containing thecoating material in dissolved form. The coating substance passes to thebase metal by diffusion and then passes into the base metal, forming agraduated alloy composition rather than a simple coating on the surfaceof the base metal. This results in protective coatings of high oxidationand corrosion resistance with good mechanical properties. Theapplication specifically discloses, among other things, the protectionof ferrous metal bases with a diffusion coating of aluminum. Whileferrous metals have heretofore been coated with aluminum by such methodsas hot dipping, the higher quality diffusion layers had not beenobtained. Aluminum diifusion coatings have been applied by so-calledpack cementation methods, wherein a part is packed with aluminum,ammonium chloride and alumina (inert filler). Upon heating, aluminumchloride vapor is formed which deposits on the part. However, uniformcoatings, particularly of edges, recesses and bores of complex shapes,are not obtained. Further, thin coatings (e.g., 1 mil or less) cannot beobtained and liquid aluminum frequently contacts the part and forms athick, non-uniform, undiifused layer. Thin coatings are particularlynecessary for applications on screws, bolts, fasteners, and the likewhere the dimensions of the final article must be strictly controlled.Aluminum provides a hard case on the surface of ferrous metals,particularly carbon or low alloy steels, which improves their behavierin oxidative and corrosive environments, as well as improves certainmechanical properties such as resistance to galling.

While the process described in the co-pending patent application leadsto high quality aluminum diffusion coatings, I have found that furthersignificant improvement can be obtained by the invention describedbelow.

An object of the present invention is an improved proc ess for providinga protective aluminum coating on ferrous metals from an alkali metalbath.

Another object is such an improved process which provides, during ashorter residence time in the bath, a diffusion coating of aluminumwhich has improved oxidation resistance.

Another object is to provide a catalyst for the bath which increases theplating rate and improves the quality of the coating.

Still another object is to control the concentration of the aluminum inthe bath and other bath conditions in a manner to obtain uniform, highquality aluminum coatings on complex shapes.

Yet another object is to provide thin, uniform diffusion coatings ofaluminum on ferrous metal bases.

The foregoing and other objects and advantages of my invention willbecome apparent from the following detailed description.

In accordance with my present invention, a diffusion coating of aluminummay be applied on ferrous metals by providing a molten bath of an alkalimetal, adding thereto aluminum and chromium, placing the ferrous basemetal in said bath, and maintaining said ferrous metal in said bathuntil an aluminum diffusion coating of desired thick ness is obtained.

I have found that the addition of chromium to the alkali metal bath,which is preferably sodium in view of its relative cheapness andsatisfactory performance, serves to accelerate the plating rate ofaluminum and greatly aids in the formation of an attractive, brightcoating of excellent oxidation and corrosion resistance. For example, a4-mil diffusion coating on carbon steel can be applied in 5 hours from asodium bath maintained at 1600 F., which time is considerably shorterthan before. Further, complex shapes can be coated with thin, evencoatings.

'The mechanism of the chromium behavior is not understood; it apparentlydoes not form part of the final aluminum diffusion coating, based uponmetallographic examination. The effect of the chromium is particularlysurprising when it is noted that diffusion coatings of chromium cannotbe applied on a ferrous metal base from a sodium bath. The concentrationof the chromium in the bath may satisfactorily vary over a broad range.For example, the concentration may be about weight percent Cr to Na,while a concentration of about 5 weight percent Cr to Na is generallyused. The chromium may be conveniently added to the bath in granular orpowder metal form.

The concentration of the aluminum in the bath is very important inproducing coatings of high quality, and I find that a lowerconcentration than previously employed considerably improves results.Thus, an aluminum concentration in the bath of about gram ofaluminumper. square inch of exposed metal surface area is highlysatisfactory while about gram per square inch surface area is aboutoptimum. Since the bath container or capsule metal is also coated withaluminum (to a like degree when such metal and the base metal are thesame), the aluminum concentration must take this into account, and theterm exposed metal surface area includes the surface area of both thebase metal to be coated and the con tainer metal. The preciseconcentration will depend upon the intended service for the coated part.For example, a thin coating (e.g., 0.10.5 mil) is useful foranti-galling applications, while a thicker coating (e.g., 1-5 mils) isdesirable for sustained oxidation resistance. Previously, in theco-pending application, concentrations of about 1 gram aluminum persquare inch surface area were employed and this could result in theformation of undesirably thick coatings without any improvements inqualities. The part to be coated should also be kept out of directcontact with solid aluminum metal, for example, by suspending the partto be coated on a screen in the bath. The contact with solid or moltenaluminum is believed to partly account for the difficulties with thepack-cementation methods.

The time-temperature relationship for my improved aluminum coatingprocess is about 1250 F.-1650 F. for a period of about %16 hours. Alonger time is required for a coating of a given thickness at a lowertemperature and the thickness of the coating will also determineresidence time in the bath. The coating thickness at a given temperatureis a function of approximately the square root of the residence time. A4-mil coating on carbon steel, for example, is obtained in about 5 hoursat 1600 F. A coating of 0.2. mil is obtained in approximately 15 minutesat 1600 F., and requires about 1-2 hours at 1250 F. I find the bestresults are obtained by the use of higher temperatures in the indicatedrange, for

3% example above 1500 F. and preferably about 1600 F., for relativelymoderate times, for example about 5 hours. However, ferrous metals whichsuffer diminishment of mechanical properties at such higher temperaturesmay be coated at lower temperatures.

I also find that, while the choice of the container material is notcritical and stainless steels may be used as in the co-pending patentapplication, nonetheless, improved results are obtained with the use ofcarbon steel containers for coating of carbon steel samples. Apparently,the alloy constituents in the stainless steels may affect the coating ofcarbon steels.

The following examples are offered to illustrate my invention in greaterdetail.

also from the inert atmosphere chamber. The parts, which were nowcovered with frozen alkali metal, were placed in butyl alcohol forcleaning.

After a distilled ater wash and drying, the tabs were visuallyinspected, remeasured, and reweighed. One of the tabs was sectioned andmounted for metallographic examination and measurement of coatingthickness. One of the tabs was immersed in hot nitric acid and itsresistance noted qualitatively. One of the tabs was suspended from astainless steel wire and placed in a box furnace at 1500 F. Weightchanges due to oxidation were determined weekly.

The following table summarizes the experimental method and the resultsof the oxidation testing for various sa u v EXAMPLES mples sin the abovemethod, and also the results of oxidation testing of an uncoated steelsample. The A 3' /2-1n. length of 1 /2-1n. outside diameter by 0.065-marked difference in protection is noted.

Table Additives to Treatment Thickness Weight Tab Wt. Gain of 60 gr. NaBath Increase to Increase Coating TabExposed to 2% Thickness to Air atTemp, Time, Tab gr. (Metallog.) 1,500 F. for A1** Cr F. Hr. Tab 500 Hr.

(interpolated) Gr. Gr. Inches Gr. Inches mg./cm 2 *Va 5 1, 500 5 .0180.285 .0120 /5 5 1, 600 5 .0192 .280 .0046 *Mo 5 1, 500 5 .0030 062 .00210. *Mo 5 1, 000 5 .0033 .051 .0032 0. 27 *lto 5 1,600 5 .0010 .024.0023 1. 00 As-recelved LOW Carbon Steel (in Hrs.) 58.00

*Tab and capsule material was 1015 low carbon steel. **Al concentrationsin terms of grams per sq. in. exposed surface area.

in. thick 1015 carbon steel tubing was welded to a As-in. thick end capof the same material. Tabs of carbon steel test material to be coatedwere ;-in. thick by approximately /z-in. wide by approximately %-in.long with a -in. diameter hole centered near one end. Commercially puresodium, 1100 aluminum sheet, and reagent grade chromium were used.

Four weighed and measured tabs were suspended on a wire frame of thesame material as the tabs. This frame was fitted in the capsule so as toplace the lower ends of the tabs approximately A in. from the bottom ofthe capsule. The capsule, which now had the bottom cap welded on, thetabs in place, and contained measured quantities of aluminum andchromium, was placed in an inert gas (argon) glove chamber.

The quantities of additives were calculated on the basis of weightpercent relative to sodium (for chromium) and on the basis of per squareinch of metal surface (tabs, wire and capsule) exposed to the bath (foraluminum).

The sodium was cleaned mechanically to remove surface oxide in the inertatmosphere and a weighed amount placed in the capsule. A small furnacein the chamber heated the capsule to facilitate filling with sodium. Theupper end cap was then welded to the capsule while in the inertatmosphere chamber.

The capsule was then withdrawn from the inert atmosphere chamber andplaced in a capsule holder which was placed in a furnace, preheated to aselected temperature. The furnace was designed to roll the capsule at 15revolutions per minute while the capsule was rocked through :30" fromthe horizontal about the transverse axis of the tube at 5 cycles perminute. Generally, four capsules were run at a time. The capsule wasprocessed in this manner for the selected time. After processing, thecapsule was removed from the furnace in the capsule holder and allowedto air-cool.

The capsule was then opened in air by sawing off the upper end cap. Thecapsule and its contents were again placed in the inert atmospherechamber, where the alkali metal (plus additive) bath was melted out. Thetabs and wi e frame were withdrawn from the capsule, and

The above examples are only illustrative rather than restrictive of myinvention. Various modifications may be made by those skilled in the artwithout departing from the spirit of my invention. Therefore, thepresent invention should be understood to be limited only as isindicated in the appended claims.

I claim:

1. An improved method of providing a diffusion coating of aluminum on aferrous metal base which comprises providing a molten sodium bath in aninert atmosphere, adding effective amounts of aluminum and chromium tosaid bath, placing said ferrous metal base material in said bath, andmaintaining said base metal in said bath until a diffusion coating ofthe desired thickness is obtained.

2. The method of claim 1, wherein approximately gram of aluminum persquare inch of exposed metal surface area is added to said bath.

3. The method of claim 1, wherein chromium is added to said bath in anamount approximately 1 weight percent Cr to alkali metal.

4. A method of providing an improved diffusion coating of aluminum on aferrous metal base which comprises providing a molten sodium bath in aninert atmosphere, dissolving in said bath approximately 5 gram ofaluminum per square inch of exposed metal surface area, adding chromiumto said bath in an amount approximately weight percent Cr to Na,positioning said ferrous metal base in said bath, and maintaining saidbase metal in said bath at a temperature of approximately 1250-1650 F.for approximately it-16 hours.

5. The method of claim 4 wherein said ferrous metal is a carbon steel.

6. The method of claim 4 wherein said temperature is maintained betweenabout 1500 F. and 1650 F.

7. A method of providing an improved diffusion coating of aluminum on acarbon steel base which comprises providing a molten sodium bath in aninert gas atmosphere, dissolving approximately gram, aluminum per squareinch surface area of exposed metal surface area, adding chromium to saidbath in an amount approxi- 5 mately A Weight percent Cr to Na, placingsaid car- 2,917,818 bon steel in said bath, and maintaining said bath at21 2,929,740 temperature of about 1600 F. for about 5 hours. 3,073,7203,085,028 References Cited by the Examiner 5 3,096,205

UNITED STATES PATENTS 2,774,686 12/56 Hodge 1171 14 2,848,352 8/58Noland et a1. 117114 Thomson 29-1962 Logan. Mets 117131 Logan 1171 14DeGuisto 29196.2

JOSEPH B. SPENCER, Primary Examiner.

RICHARD D. NEVIUS, Examiner.

1. AN IMPROVED METHOD OF PROVIDING A DIFFUSION COATING OF ALUMINUM ON AFERROUS METAL BASE WHICH COMPRISES PROVIDING A MOLTEN SODIUM BATH IN ANINERT ATMOSPHERE, ADDING EFFECTIVE AMOUNTS OF ALUMINUM AND CHROMIUM TOBATH, AND MAINTAINING SAID BASE METAL IN SAID BATH UNTIL A DIFFUSIONCOATING OF THE DESIRED THICKNESS IS OBTAINED.